1. Field of the Invention
Intruder alarm systems.
2. Description of the Prior Art
In intruder alarm systems in which acoustic or other vibrations arising fron the movements of an intruder within a protected area are sensed by one or more electromechanical transducers which provide electric signals from which an alarm condition may be recognised, it is necessary reliably to distinguish vibrations caused by a human intruder from those from other sources such as rain or hail, small animals, tree roots or nearby road, rail or air traffic, so as to avoid too many false alarms.